Post inspector related worker tasks to a separate queue

Instead of using one queue with multiple modes, we use two queues. For each
inspector task, we post a tickle task to the main queue that will execute one
inspector task from the inspector queue.

If the worker is paused, tasks from the inspector queue are directly executed.

Also rip out the predicate support from MessageQueue as it's no longer used.

BUG=301514
R=yurys@chromium.org,pfeldman@chromium.org,kinuko@chromium.org

Review URL: https://codereview.chromium.org/163343003

git-svn-id: svn://svn.chromium.org/blink/trunk@167317 bbb929c8-8fbe-4397-9dbb-9b2b20218538